Multilayer optical disc

ABSTRACT

A multilayer optical disc which has three or more recording layers and enables easy positioning of a focused beam spot onto a particular recording layer in which a BCA is disposed. An inter-layer distance between a particular recording layer and a recording layer adjacent to the particular recording layer is larger than the other inter-layer distances in which, at the focused beam spot positioning, the focused beam spot traverses the said adjacent recording layer earlier than the other recording layer adjacent to the particular recording layer.

This application claims the priority date of Jun. 1, 2007 on whichJapanese Patent Application 2007-146435 was filed, the disclosure ofthat Japanese Application being herein incorporated by reference.

TECHNICAL FIELD

The present invention relates to multilayer optical discs having threeor more recording layers.

BACKGROUND ART

As measures to increase the storage capacity of an optical disc, therehave been known use of laser beams having a shorter wavelength forrecording/reproduction of data, reduction of the size of laser beamspots by the use of objective lenses having a higher numerical aperture,use of multilayer discs having plural recording layers, and others. Asfor the multilayer disc, two-layered discs have already been put intopractical use for the DVDs and Blu-ray discs (hereafter, referred to as“BD”).

Recently, optical discs having three or more recording layers have beenproposed, and specifically, BDs having six recording layers have beenproposed (see, for example, ODS2006 Technical digest(2006)041).

DISCLOSURE OF INVENTION

Here, consideration will be given to a recording layer onto which afocused beam spot is to be first positioned.

The two-layered BD has a recording layer called L0, which is at a depthof 100 μm from a disc surface on which a laser beam is to be incidentfor recording/reproduction of information, and the other recording layercalled L1, which is at a depth of 75 μm from the disc surface. The BCA(Bust Cutting Area) code including DI (Disc Information) carryinginformation on the disc category, etc. is disposed in the recordinglayer L0.

FIG. 2 is a diagrammatic view of an optical disc having a BCA.

Referring to FIG. 2, an optical disc 201 has a hole 202 formed at acenter of the disc for use in the mounting of the disc, and a BCA 203 isdisposed around the hole 202. With the optical disc 201 being rotated,by operating a focusing servo onto the recording layer L0 of the disc atthe diametrical position of the BCA, levels of light reflected from thedisc 201 will produce a bar code data containing a repetition of highand low intensities. This bar code data represents a BCA code includingthe DI.

For reading out the BCA code, the focusing servo has only to beperformed, while the tracking servo is unnecessary therefor. The opticaldisc apparatus discriminates, by the use of levels of reflection signalsor the like, in which category of medium among such as BD-ROM, BD-R,etc. a mounted optical disc is, with the final determination of themedium category being carried out by referring to the DI recorded inadvance in the optical disc 201. In order to complete the determinationof the category of medium in a short time, it is necessary to positionthe focused beam spot onto a recording layer where the BCA 203 isdisposed, i.e., onto a recording layer in which information to be firstreproduced is recorded. Therefore, it is desirable that, with atwo-layered BD, the optical disc apparatus should first position thefocused beam spot onto the recording layer LO of the disc.

With respect to the six-layered BD, since no standards have beenestablished yet therefor, it is unknown in which of the layers the BCAor a similar one should be disposed. Assuming that the conventionalapproach is taken, it is conjectured that the BCA or the like may bedisposed in the recording layer L0 in the six-layered BD disclosed inthe above-mentioned literature which layer is located the deepest in theBD. In that case, it is desirable that an optical disc apparatussupporting six-layered BDs should first position the focused beam spotonto the deepest located recording layer L0, i.e., onto the recordinglayer in which information to be first reproduced is recorded.

An object of the present invention is to a multilayer optical dischaving three or more recording layers in which it is possible to easilyposition a focused beam spot onto a recording layer in which informationto be first reproduced is recorded.

An object of the present invention may be, for example, accomplished byadjusting distances between each of recording layers in whichinformation to be first reproduced is recorded and other recordinglayers adjacent thereto.

According to the present invention, in a multilayer optical disc havingthree or more recording layers, it is possible to easily position afocused beam spot onto a recording layer in which information to befirst reproduced is recorded.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described hereinafter.

Embodiment 1

Embodiment 1 of the present invention will now be described.

With reference to the diagrammatic view of FIG. 3, an operation ofpositioning a focused beam spot and signal waveforms will be explainedin connection with a six-layered BD. In FIG. 3, it is assumed that themechanism for the spherical aberration correction provided in theoptical disc apparatus is adjusted in advance so as to assume an optimumwith respect to the recording layer L0.

In FIG. 3, view (a) shows a cross section of a six-layered BD in whichthe arrow represents a locus of a laser beam spot when an objective lensis moved upward for the positioning of a focused beam spot, while view(b) conceptually shows a focus error (hereafter, referred to as “FE”)signal in which a well-known S-curve signal zero-crossing at time T0appears when the laser beam spot traverses the disc surface. With theobjective lens being further moved up, another S-curve signalzero-crossing at time T1 appears on the FE signal when the laser beamspot traverses the recording layer L5. In the similar manner, S-curvesignals appear at times T2, T3, T4, T5 and T6 on the FE signal when thelaser beam spot traverses the recording layers L4, L3, L2, L1 and L0,respectively.

It is known that a multilayer optical disc suffers spherical aberrationproblems stemming from differences between distances between the discsurface on which a laser beam for recording/reproducing information isincident and the respective recording layers so that the amplitudes ofS-curve signals corresponding to the respective recording layers aredifferent. For example, in FIG. 3, since the aberration correction hasbeen set so as to be optimum with respect to the recording layer L0, theamplitude of the S-curve signal corresponding to the recording layer L0is larger than those of the S-curve signals corresponding to the otherrecording layers. Since the aberration correction with respect to therecording layer L1 is deviated from the optimum value, the amplitude ofthe S-curve signal corresponding to the recording layer L1 is smallerthan that of the S-curve signal corresponding to the recording layer L0.In the similar manner, the amplitudes of the S-curve signals on the FEsignal corresponding to the recording layers L2, L3, L4 and L5 arebecoming smaller as they are farther from the recording layer L0.

Owing to this phenomenon, as shown in FIG. 3, it is possible toaccurately position the focused beam spot onto the recording layer L0 byestablishing a detection level Vth between the bottom peak levels B1 andB0 of the respective S-curve signals corresponding to the recordinglayers L1 and L0, and detecting a zero-crossing of the FE signal afterthe FE signal has exceeded the detection level Vth.

In order to adopt the above approach, it is indispensable that theS-curve signal amplitude corresponding to the recording layer L0 inwhich the BCA is disposed is the largest. However, the S-curve signalamplitudes of the respective recording layers may be varied due to thereflectivities of the respective recording layers and errors in thespherical aberration correction, which may lead to problems such thatthe bottom level of the S-curve signal corresponding to a recordinglayer other than the layer L0 exceeds the detection level Vth or thebottom level of the S-curve signal corresponding to the recording layerL0 does not exceed the detection level Vth. Therefore, to solve suchproblems, it is necessary to provide an optical disc in which theS-curve signal amplitude corresponding to the recording layer L0 is thelargest.

FIG. 1 is a diagrammatic sectional view of a six-layered optical discaccording to Embodiment 1 of the present invention.

Reference numeral 100 represents a cover layer having a thickness of 40μm and made of a transparent resin or the like.

Reference numerals 101 to 106 represent recording layers L5, L4, L3, L2,L1 and L0, respectively. It is assumed that all of these recordinglayers have the same reflectivity. Each of the recording layers is madeof a phase-change material or an organic material and is in a laminationstructure. However, since such lamination structure does not have to dowith this embodiment, description thereof will be omitted.

Reference numerals 107 to 111 represent space layers. The space layers107 to 110 have a thickness of 15 μm, while the space layer 111 has athickness of 20 μm. Each of the space layers is made of a transparentresin.

Reference numeral 112 represents a substrate made of a polycarbonate orthe like.

Further, the overall thickness of the optical disc including the coverlayer 100 to the substrate 112 is 1.2 mm.

The laser beam for recording/reproducing information is incident on thecover layer 100.

The optical disc according to this embodiment is featured in that thespace layer 111 has a thickness larger than those of the other spacelayers. Resultant effects will be described with reference to FIG. 4.

FIG. 4 is a diagrammatic view showing relations between the correctionsof spherical aberration and the S-curve signal amplitudes on the FEsignal. In FIG. 4, curve (a) is obtained by plotting the S-curve signalamplitudes corresponding to the recording layer L5 with changes of thespherical aberration correction. Since the distance from the discsurface to the recording layer L5 is 40 μm, its optimum sphericalaberration correction is 40 μm, with which the S-curve signal amplitudeis maximum. Curve (b) is obtained by plotting the S-curve signalamplitudes corresponding to the recording layer L4 with changes of thespherical aberration correction. Since the distance from the discsurface to the recording layer L4 is 55 μm which is a sum of thethicknesses of the cover layer 100 and the space layer 107, its optimumspherical aberration correction is 55 μm, with which the S-curve signalamplitude is maximum. Likewise, curves (c), (d), (e) and (f) areobtained by plotting the S-curve signal amplitudes corresponding to therecording layers L3, L2, L1 and L0, respectively, with changes of thespherical aberration correction. The S-curve signal amplitudescorresponding to these recording layers are maximum with their sphericalaberration corrections of 70 μm, 85 μm, 100 μm and 120 μm, respectively,which are equal to the distances from the disc surface to the respectiverecording layers.

In this connection, since the reflectivities of the respective recordinglayers are the same, as mentioned above, and the maximum values of therespective S-curve signal amplitudes are equal to one another, themaximum amplitude value is assumed to be 100%.

When the spherical aberration correction with respect to the recordinglayer L0 is 120 μm, the difference between the S-curve signal amplitudescorresponding to the recording layers L0 and L1 corresponds to D1 inFIG. 4. Thus, the level difference between the bottom levels B1 and B0of the S-curve signals shown in FIG. 3 corresponds to about one-half ofthe above-mentioned difference D1.

A comparison of effects will now be made between this embodiment and acase where this embodiment is not applied. As an example of the case inwhich the present embodiment is not applied, it is presumed that thespace layer 111 between the recording layers L1 and L0 is 15 μm which isequal to the thicknesses of the other space layers. The relation betweenthe spherical aberration correction and the S-curve signal amplitudeplotted under these conditions is represented by dotted curve (g) inFIG. 4. With the space layer 111 having a thickness of 15 μm, thethickness as measured from the disc surface to the recording layer L0 is115 μm, and therefore, the curve (g) is maximum when the sphericalaberration correction is 115 μm. In this case, the difference betweenthe S-curve signal amplitudes corresponding to the recording layers L0and L1 will correspond to D2 as in FIG. 4.

As is clear from FIG. 4, the S-curve signal amplitude difference D1according to this embodiment is greater than the S-curve signalamplitude difference D2 in the case to which this embodiment is notapplied. This means that the difference between the bottom levels B1 andB0 in FIG. 3 is made larger with a result that it is possible toincrease the margin for the threshold level Vth for the respectivebottom levels B0 and B1. As a result, even though the S-curve signalamplitudes on the FE signal are varied due to errors in thereflectivities of the recording layers L0 and L1 or errors in thethickness of the space layer 111, it will no longer occur that thebottom level B0 of the S-curve signal in FIG. 3 is higher than thethreshold level Vth or the bottom level B1 of the S-curve signal in FIG.3 is lower than the threshold level Vth. Thus, it is possible toaccurately detect the S-curve signal corresponding to the recordinglayer L0 by means of the threshold level Vth, which in turn makes itpossible to correctly position a focused beam spot onto a desiredrecording layer L0.

In Embodiment 1 of the present invention described above, itcontemplates making the thickness of the space layer 111 between theparticular recording layer L0 onto which to position a focused beam spotand the recording layer L1 larger than the thicknesses of the otherspace layers to thereby increase the bottom level difference between thefocus S-curve signals corresponding to the recording layers L0 and L1.By this, the optical disc apparatus is capable of accurate detection ofthe S-curve signal corresponding to the recording layer L0 with a resultthat correct positioning of a focused beam spot onto the intendedrecording layer L0 becomes possible.

Although in Embodiment 1 above, a structure is described in which a BCAis disposed in the recording layer L0, the recording layer in which todispose a BCA is not limited thereto. For example, a BCA may be disposedin the recording layer L2, and in that case the intended layer ontowhich to position a focused beam spot will be L2. In that case, thethickness of the space layer 109 between the recording layers L2 and L3should be made larger than the thicknesses of the other space layers,whereby accurate positioning of a focused beam spot on the recordinglayer L2 is possible in the same manner as in the operation describedabove.

Embodiment 2

In the structure of the six-layered optical disc according to Embodiment1, a BCA is disposed in the recording layer L0. However, the BCA may bedisposed in the recording layer L5.

Further, in the description of Embodiment 1, positioning of a focusedbeam spot is carried out by moving the objective lens upward. Meanwhile,it is known that, in order to avoid collision of the objective lens withthe disc surface in case of failure in the focused beam spotpositioning, the objective lens is once moved upward to such an extentthat a laser beam spot have traversed all of the recording layers andthereafter, to attain the focused beam spot positioning, the objectivelens is moved downward.

In Embodiment 2, a multilayer optical disc will be described in which aBCA is disposed in the recording layer L5 in a six-layered optical disc,and with this multilayer optical disc it possible to accurately positiona focused beam spot onto the recording layer L5 in the course of theabove-mentioned downward focused beam spot positioning.

FIG. 5 is a diagrammatic cross-sectional view of a six-layered opticaldisc according to Embodiment 2 of the present invention. Constituentelements similar to those in FIG. 1 are represented by the samereference numerals and description thereof will be omitted.

Reference numerals 113 and 114 represent space layers. The space layer113 is 15 μm thick, while the space layer 114 is 20 μm thick.

As in Embodiment 1, the overall thickness of the optical disc includingthe cover layer 100 to the substrate 112 is 1.2 mm, and the laser beamis incident on the cover layer 100.

The optical disc according to this embodiment is featured in that thespace layer 114 has a thickness larger than those of the other spacelayers. Resultant effects will be described with reference to FIG. 6.Since the various members shown in FIG. 6 are similar to those shown inFIG. 3, description thereof will be omitted.

The optical disc apparatus has been adjusted in advance so that thespherical aberration correction assumes an optimum with respect to therecording layer L5. Under this condition, when an objective lens ismoved upward, the laser beam spot traverses the disc surface at time TO,and further traverses the recording layers L5, L4, L3, L2, L1 and L0 attimes T1, T2, T3, T4, T5 and T6, respectively, as shown in FIG. 6. Attime T7 which is later than the time of traversal of the recording layerL0 by the laser beam spot, the objective lens is changed over to andownward movement, so that the objective lens moving downward traversesthe recording layers L0, L1, L2, L3, L4 and L5 at times T8, T9, T10,T11, T12 and T13, respectively.

In accordance with the operation of the objective lens described above,as the laser beam spot traverses the respective recording layers,S-curve signals appear on the FE signal. Here, since the sphericalaberration correction has been set so as to be optimum with respect tothe recording layer L5, which is a target for the focused beam spotpositioning, the S-curve signal amplitudes at the traversal of therecording layer L5 at times T1 and T13 are maximum.

Here, in Embodiment 2, since the thickness of the space layer 114between the recording layers L5 and L4 is larger than those of the otherspace layers, owing to the functional effect similar to that describedabove with reference to FIG. 4 in connection with Embodiment 1, thedifference between the top level B3 of the S-curve signal correspondingto the recording layer L4 appearing around time T12 and the top level ofB2 of the S-curve signal corresponding to the recording layer L5appearing around time T13 is made larger with a result that it ispossible to increase the margin for the threshold level Vth for therespective bottom levels B2 and B3. As a result, even though the S-curvesignal amplitudes on the FE signal are varied due to errors in thereflectivities of the recording layers L5 and L4 or errors in thethickness of the space layer 114, it will no longer occur that the toplevel B2 of the S-curve signal in FIG. 6 is lower than the thresholdlevel Vth or the top level B3 of the S-curve signal in FIG. 6 is higherthan the threshold level Vth. Thus, it is possible to accurately detectthe S-curve signal corresponding to the recording layer L5 by means ofthe threshold level Vth, which in turn makes it possible to correctlyposition a focused beam spot onto the particular recording layer L5,provided that the focus servo loop is turned on at time T13 when the FEsignal zero-crosses after the S-curve signal corresponding to therecording layer L5 has been detected.

In Embodiment 2 of the present invention described above, itcontemplates making the thickness of the space layer 114 between theparticular recording layer L5 onto which to position a focused beam spotand the recording layer L4 larger than the thicknesses of the otherspace layers to thereby increase the bottom level difference between thefocus S-curve signals corresponding to the recording layers L5 and L4.By this, the optical disc apparatus is capable of accurate detection ofthe S-curve signal corresponding to the recording layer L5 with a resultthat correct positioning of a focused beam spot onto the intendedrecording layer L5 is possible.

Although in Embodiment 2 above, a structure is described in which a BCAis disposed in the recording layer L5, the recording layer in which todispose a BCA is not limited thereto. For example, a BCA may be disposedin the recording layer L3, and in that case the intended layer ontowhich to position a focused beam spot will be L3. In that case, thethickness of the space layer 109 between the recording layers L2 and L3should be made larger than the thicknesses of the other space layers,whereby correct positioning of a focused beam spot on the recordinglayer L3 is possible in the same manner as in the operation describedabove.

In Embodiment 1 and Embodiment 2, the cover layer is 40 μm thick and therespective space layers are 15 μm or 20 μm thick. However, thethicknesses of the the layers should not be limited thereto, and theymay be optionally determined so as to provide desirable discrecording/reproducing performances.

Further, in Embodiment 1 and Embodiment 2, six-layered optical discs aredescribed by way of example, but there is no need to say that theteaching of the present invention is also applicable to multilayeroptical discs having three or more layers.

Further, in Embodiment 1 and Embodiment 2, recordable multilayer discsare described, but the teaching of the present invention is alsoapplicable to read-only multilayer optical discs.

Further, in Embodiment 1 and Embodiment 2, a structure is described inwhich the disc category information is contained in a BCA code, butthere is no need to place the limitation to BCA as long as the disccategory information is involved.

Although the present invention has been described with respect to someembodiments, it is evident to those skilled in the art that the presentinvention should not be restricted thereto and various changes andmodifications are possible within the spirit of the present inventionand the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a six-layered optical disc showingEmbodiment 1 of the present invention.

FIG. 2 is a diagrammatic view of an optical disc having a BCA.

FIG. 3 is a diagrammatic view of a focus error signal waveform observedwhen an objective lens is moved upward.

FIG. 4 is a diagrammatic view showing relations between the correctionsof spherical aberration and the focusing S-curve signal amplitudes.

FIG. 5 is a sectional view of a six-layered optical disc showingEmbodiment 2 of the present invention.

FIG. 6 is a diagrammatic view showing a focus error signal waveformobserved when an objective lens is moved upward and downward.

1. A multilayer optical disc having at least three recording layers,wherein an inter-layer distance between a first recording layer and asecond recording layer is larger than another inter-layer distance,information to be first reproduced being recorded in said firstrecording layer, and said second recording layer being nearest, amongrecording layers adjacent to said first recording layer, to a discsurface on which a laser beam for recording/reproducing information isto be incident.
 2. A multilayer optical disc according to claim 1,wherein said first recording layer is a recording layer farthest fromsaid disc surface on which the laser beam for recording/reproducinginformation is to be incident.
 3. A multilayer optical disc according toclaim 1, wherein said first recording layer is a recording layer havinga BCA.
 4. A multilayer optical disc having at least three recordinglayers, wherein an inter-layer distance between a first recording layerand a second recording layer is larger than another inter-layerdistance, information to be first reproduced being recorded in saidfirst recording layer, and said second recording layer being farthest,among recording layers adjacent to said first recording layer, from adisc surface on which a laser beam for recording/reproducing informationis to be incident.
 5. A multilayer optical disc according to claim 4,wherein said first recording layer is a recording layer nearest to saiddisc surface on which on which a laser beam for recording/reproducinginformation is to be incident.
 6. A multilayer optical disc according toclaim 4, wherein said first recording layer is a recording layer havinga BCA.
 7. A multilayer optical disc according to claim 2, wherein saidfirst recording layer is a recording layer having a BCA.
 8. A multilayeroptical disc according to claim 5, wherein said first recording layer isa recording layer having a BCA.